Meningococcal sepsis results partly from overproduction of host cytokines after macrophages interact with endotoxin. Using the D-galactosamine-sensitized mouse model for determining endotoxin lethality, we found that the toxicity of purified lipooligosaccharide (LOS) from M986, a group B disease strain, was 3 to 4 times higher than that of purified LOS from noncapsulated M986-NCV-1 and OP-, an truncated LOS mutant. Outer membrane vesicles (OMVs) and detergent-treated OMVs (D-OMVs) from the three strains were 2 to 3 and over 300 times less toxic than the purified LOSs, respectively. Intraperitoneal administration of these preparations induced serum TNF-a and IL-6 two hours after injections. However, repeated doses of low- and high-toxicity preparations induced lower amounts of TNF-a and IL-6, i.e., LOS tolerance. Peritoneal macrophages from tolerant mice pretreated with either high- or reduced-toxicity LOS preparations produced only a fraction of TNF-a and IL-6 compared with control groups in response to LOS ex vivo. Despite tolerance to LOS induced by pretreatment with reduced-toxicity preparations, killing of N. meningitidis M986 by macrophages from these animals was enhanced. Protection was achieved when mice treated with LOS and, especially, D-OMV were challenged with live N. meningitidis. The least toxic LOS in D-OMV was most effective in inducing hyporesponsiveness to endotoxin in mice but protected them against challenge with N. meningitidis. Thus detergent treatment of OMVs can drastically reduce toxicity while maintaining or even enhancing the immunological stimulatory effects of LOS.